In the field of telemetry it is often necessary to monitor the frequency of a periodic signal and to detect any frequency variations. Such monitoring of a low-frequency signal is conventionally implemented by first measuring the period of the signal and then converting this measurement to a frequency value. Generally, a high-frequency pulse train is fed as a stepping signal to a counter whose operations are started and stopped by pulses derived from zero-crossings of consecutive cycles of the low-frequency periodic signal. The pulses counted by the counter are proportional in number to the duration of the period of the low-frequency signal. An advantage of this measurement is the reduction of error arising from an uncounted pulse. For example, in directly measuring the frequency of a 10-Hz sinusoidal signal by counting the number of pulses derived from similarly sloped zero crossings during a one-second interval, the error due to missing a pulse is ten percent, whereas in measuring the period of the 10-Hz signal by counting the number of pulses arriving from a 1-MHz oscillator during a 0.1-second interval, the error due to miscounting a pulse is only 0.001 %.
Such high accuracy, however, is rarely attained in practice, because the zero crossings of the sinusoidal signal do not recur at identical intervals. This phenomenon, observable as a "jitter" of the signal along the time axis of an oscillograph output, is statistical in nature and arises as a result of random interference voltages harmonically unrelated to the sinusoidal signal being monitored. Telemetry systems frequently have an interference-voltage range of forty decibels, which introduces into a periodic signal a jitter having a magnitude equal to 0.1% of the signal's period. Such a jitter increases the error of the above-described period measurement method by a factor of a hundred.
Devices are known in which the effects of jitter on the accuracy of period measurement are reduced by several orders of magnitude. Most of these devices calculate an average period by counting stepping pulses during n consecutive cycles of the monitored signal and dividing the resulting sum by n. For example, a counter whose contents are incrementable by a high-frequency pulse train is enabled by a pulse derived from a cycle of the monitored signal and is later disabled by a pulse derived from the hundredth subsequent cycle of this signal. The counter contents upon disabling are proportional to the hundred-cycle interval. The effects of jitter, however, are the same as for a single cycle. Thus, upon division of the counter contents by 100, jitter-induced error is reduced by the same factor.
A disadvantage of period-averaging devices of this kind is the delay involved. For instance, the averaging of a 10-Hz signal over 100 cycles requires 10 seconds. Even longer delays may be necessary, depending on the magnitudes of the interference voltages and the accuracy requirements. In the particular case of monitoring shaft rotation, conventional devices are only limitedly useful.
Measurement accuracy can be raised through the use of circuitry inserted upstream of the period counter for decreasing zero-crossing shifts due to jitter. Such a solution has the inherent disadvantage of being restricted to narrow frequency bands.